The introduction and availability of products which are produced in an economically and environmentally sustainable manner are currently of great interest commercially. The growing political pressure to reduce the consumption of fossil fuels has increased the production and consumption of alternative fuels such as those manufactured from renewable resources, in particular biodiesel. Biodiesel comprises methyl esters of long-chain fatty acids and is produced from vegetable oils (triglycerides) by a trans-esterification reaction with methanol. A problem with the increased consumption of biodiesel is that large amounts of land are required to grow the oil-bearing crops from which the vegetable oil is produced. When the biodiesel crops are grown on land which has been illegally cleared or diverted from food production then its production may have undesirable consequences for the environment and biodiversity. There is therefore a need to provide a method of identifying vegetable oils which have been produced in a sustainable manner. It is an object of the invention to provide such a method.